1. Field of the Invention
This invention relates to a fuel injection control system for an internal combustion engine of an in-cylinder fuel injection type in which fuel is directly injected into a combustion chamber within each cylinder, the fuel injection control system controlling a fuel injection time period to thereby control the amount of fuel injected into the cylinder.
2. Description of the Prior Art
Conventionally, a fuel injection control system of the above-mentioned kind was proposed e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 7-166922. In the proposed fuel injection control system, fuel behavior parameters indicative of behaviors of fuel injected into a combustion chamber and a fuel injection amount Fi are determined in the following manner: A combustion-chamber fuel deposition ratio ac is retrieved from a map according to the engine temperature TE and the engine rotational speed NE. Further, a cylinder fuel deposition ratio ad is determined based the engine temperature TE and the engine coolant temperature TW, and a gaseous fuel ratio xcex2c is calculated from the combustion-chamber fuel deposition ratio ac and the cylinder fuel deposition ratio ad.
Then, a fuel evaporation ratio xcex3c is retrieved from a map according to the engine temperature TE and the engine rotational speed NE, and an exhaust carry-off ratio xcex3e is calculated from the fuel evaporation ratio xcex3c. Further, a gaseous fuel residual ratio xcex2k is retrieved from a map according to the engine temperature TE and the engine rotational speed NE. Then, the present value of a combustion-chamber gaseous fuel amount Fc is calculated from the engine rotational speed NE and an intake air amount Q. The fuel injection amount Fi is calculated from the present value of the combustion-chamber gaseous fuel amount Fc, the aforementioned fuel behavior parameters, and the immediately preceding value of a combustion-chamber deposited-fuel amount Mc. The present value of the combustion-chamber deposited-fuel amount Mc is also calculated from the fuel behavior parameters, the immediately preceding value of the combustion-chamber deposited-fuel amount Mc, and the fuel injection amount Fi. Further, a fuel injection time period Tout is calculated from the fuel injection amount Fi.
On the other hand, an in-cylinder fuel injection engine is known in which when the engine is operating under a very low load, such as idling, fuel is injected during a compression stroke to cause a mixture having a far leaner air-fuel ratio than the stoichiometric air-fuel ratio to be distributed in a combustion chamber and burned by stratified combustion. In this kind of engine, since fuel injection is carried out during the compression stroke, fuel is supplied via a fuel injection valve in a state pressurized by a fuel pump to a level much higher than when fuel injection is carried out during an intake stroke. Further, the pressure of fuel within the fuel injection valve has a characteristic that it drops temporarily immediately after fuel injection via the fuel injection valve and then returns to a predetermined level, and hence the actual fuel pressure tends to change largely. Further, the actual fuel pressure also tends to change largely according to changes in the discharge pressure of the fuel pump.
In the above conventional fuel injection control systems, the fuel injection time period Tout, the fuel behavior parameters, and the combustion-chamber gaseous fuel amount Fc, and the combustion-chamber deposited-fuel amount Mc are calculated by using the engine temperature TE, the engine rotational speed NE, the engine coolant temperature TW, and the intake air amount Q as parameters. However, even if the fuel injection time period Tout is held constant, due to changes in fuel pressure, the amount of fuel actually injected into each cylinder deviates from a desired amount. Particularly in the case of the above in-cylinder fuel injection engine in which stratified combustion is performed, fuel pressure changes largely, so that the deviation (error) of the actual fuel injection amount from the desired amount tends to be increased. Fuel pressure is closely related to fuel deposition in a combustion chamber, and hence a parameter, such as the combustion-chamber deposited-fuel amount Mc, varies with the fuel pressure. However, the conventional fuel injection control systems is incapable of calculating the combustion-chamber deposited-fuel amount Mc such that the combustion-chamber deposited-fuel amount Mc reflects the fuel pressure, and hence incapable of properly calculating the combustion-chamber deposited-fuel amount Mc and the fuel injection amount Fi. Particularly in the case of the in-cylinder fuel injection engine in which stratified combustion is performed, fuel pressure undergoes large changes, and hence an error in calculation of the fuel injection amount Fi tends to be increased. For these reasons, e.g. in the execution of feedback control of the air-fuel ratio, the fuel injection amount cannot be properly calculated according to a desired air-fuel ratio, which degrades convergence of the air-fuel ratio control.
It is an object of the invention to provide a fuel injection control system for an internal combustion engine of an in-cylinder fuel injection type, which is capable of properly determining a fuel injection time period such that the fuel injection time period reflects a fuel pressure and a deposition of fuel, thereby accurately controlling the amount of fuel actually injected.
To attain the above object, the present invention provides a fuel injection control system for an internal combustion engine of an in-cylinder fuel injection type in which fuel is directly injected into a combustion chamber within a cylinder, the fuel injection control system controlling a fuel injection time period to thereby control an amount of fuel to be injected into the cylinder.
The fuel injection control system according to the invention is characterized by comprising:
operating condition-detecting means for detecting operating conditions of the engine;
required fuel amount-determining means for determining a required fuel amount based the detected operating conditions;
deposited-fuel amount-determining means for determining a deposited-fuel amount of fuel which is deposited within the combustion chamber, based the detected operating conditions;
required fuel amount-correcting means for correcting the required fuel amount according to the determined deposited-fuel amount;
fuel pressure-detecting means for detecting a fuel pressure of fuel to be injected into the cylinder; and
fuel injection time period-determining means for determining the fuel injection time period as a time period equivalent to a value obtained by correcting the corrected required fuel amount according to the detected fuel pressure.
According to this fuel injection control system, the required fuel amount and the deposited-fuel amount are each determined based on operating conditions of the engine, and the required fuel amount is corrected according to the deposited-fuel amount. Then, the corrected required fuel amount is further corrected according to the fuel pressure, thereby determine the fuel injection time period. Since the fuel injection time period is thus determined by correcting the required fuel amount corrected according to the deposited-fuel amount, according to the fuel pressure, it is possible to properly determine the fuel injection time period such that the fuel injection time period reflects the fuel pressure and the deposited fuel. This makes it possible to accurately control the amount of fuel actually injected into the cylinder. As a result, e.g. in the execution of feedback control of the air-fuel ratio, convergence of the feedback control can be enhanced to thereby improve exhaust emission characteristics.
Preferably, the fuel injection control system further includes fuel behavior parameter-calculating means for calculating fuel behavior parameters indicative of behaviors of fuel injected into the cylinder, based on the detected operating conditions, and fuel behavior parameter-correcting means for correcting the fuel behavior parameters according to the detected fuel pressure, the deposited-fuel amount-determining means determining the deposited-fuel amount based on the fuel behavior parameters corrected by the fuel behavior parameter-correcting means.
According to this preferred embodiment, the fuel behavior parameters are calculated based on the operating conditions of the engine, and then corrected according to the fuel pressure. Further, the deposited-fuel amount in the combustion chamber is determined according to the corrected fuel behavior parameters, and then the fuel injection amount is determined according to the corrected deposited-fuel amount. The fuel pressure and the deposition of fuel in the combustion chamber are closely related to each other, and the deposited-fuel amount varies with the fuel pressure. Therefore, by using the fuel behavior parameters corrected according to the fuel pressure, it is possible to properly determine the deposited-fuel amount such that the deposited-fuel amount reflects the fuel pressure, and thereby properly determine the fuel injection amount. As a result, e.g. in the execution of feedback control of the air-fuel ratio, convergence of the feedback control can be enhanced, which makes it possible to improve exhaust emission characteristics.
Preferably, at least one of an intake valve and an exhaust valve of the internal combustion engine can be selectively switched to any one of a plurality of valve timings, and the fuel behavior parameter-correcting means corrects the fuel behavior parameters according to the selected valve timing.
According to this preferred embodiment, since the fuel behavior parameters are corrected according to a selected valve timing, it is possible to determine the fuel injection amount properly in response to changes in the amount of intake air, the timing of fuel injection, etc. caused by the switching of the valve timing.
More preferably, the operating condition-detecting means includes crank angle position-detecting means for detecting a crank angle position of the cylinder, and the fuel injection amount-determining means includes differential pressure-calculating means for calculating a differential pressure between the detected fuel pressure and a pressure within the cylinder which is estimated from the detected crank angle position of the cylinder, the value being obtained by correcting the corrected required fuel amount according to the differential pressure.
More preferably, the parameters indicative of behaviors of fuel are a ratio of an amount of part of injected fuel which is injected in the present combustion cycle, the part of the injected fuel being actually burned in the present combustion cycle, to an amount of the injected fuel, and a ratio of an amount of part of deposited fuel which was left deposited on inner walls of a combustion chamber defined within the cylinder at an end of the immediately preceding combustion cycle, the part of the deposited fuel being actually burned in the present combustion cycle, to an amount of the deposited fuel.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.